全文获取类型
收费全文 | 80956篇 |
免费 | 525篇 |
国内免费 | 384篇 |
专业分类
化学 | 26199篇 |
晶体学 | 794篇 |
力学 | 6821篇 |
数学 | 32462篇 |
物理学 | 15589篇 |
出版年
2023年 | 44篇 |
2022年 | 72篇 |
2021年 | 82篇 |
2020年 | 74篇 |
2019年 | 103篇 |
2018年 | 10479篇 |
2017年 | 10286篇 |
2016年 | 6183篇 |
2015年 | 953篇 |
2014年 | 409篇 |
2013年 | 493篇 |
2012年 | 4058篇 |
2011年 | 10778篇 |
2010年 | 5783篇 |
2009年 | 6174篇 |
2008年 | 6809篇 |
2007年 | 8945篇 |
2006年 | 425篇 |
2005年 | 1469篇 |
2004年 | 1663篇 |
2003年 | 2064篇 |
2002年 | 1095篇 |
2001年 | 282篇 |
2000年 | 327篇 |
1999年 | 177篇 |
1998年 | 204篇 |
1997年 | 159篇 |
1996年 | 204篇 |
1995年 | 119篇 |
1994年 | 83篇 |
1993年 | 103篇 |
1992年 | 59篇 |
1991年 | 69篇 |
1990年 | 57篇 |
1989年 | 69篇 |
1988年 | 65篇 |
1987年 | 67篇 |
1986年 | 64篇 |
1985年 | 54篇 |
1984年 | 46篇 |
1983年 | 44篇 |
1982年 | 50篇 |
1981年 | 45篇 |
1980年 | 51篇 |
1979年 | 49篇 |
1914年 | 45篇 |
1913年 | 40篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
21.
Markus Bödenler Ludovic de Rochefort P. James Ross Nicolas Chanet Geneviève Guillot Gareth R. Davies 《Molecular physics》2019,117(7-8):832-848
ABSTRACTFast field-cycling (FFC) nuclear magnetic resonance relaxometry is a well-established method to determine the relaxation rates as a function of magnetic field strength. This so-called nuclear magnetic relaxation dispersion gives insight into the underlying molecular dynamics of a wide range of complex systems and has gained interest especially in the characterisation of biological tissues and diseases. The combination of FFC techniques with magnetic resonance imaging (MRI) offers a high potential for new types of image contrast more specific to pathological molecular dynamics. This article reviews the progress in FFC-MRI over the last decade and gives an overview of the hardware systems currently in operation. We discuss limitations and error correction strategies specific to FFC-MRI such as field stability and homogeneity, signal-to-noise ratio, eddy currents and acquisition time. We also report potential applications with impact in biology and medicine. Finally, we discuss the challenges and future applications in transferring the underlying molecular dynamics into novel types of image contrast by exploiting the dispersive properties of biological tissue or MRI contrast agents. 相似文献
22.
23.
24.
25.
The structure, chemical composition, and magnetic properties of electrochemically deposited nanocrystalline Co-Ni-Fe films
were investigated using a number of techniques. A high saturation magnetic induction up to B
s
= 21 kG was attained. An enhancement of the saturation magnetization compared to the ideal anticipated one was revealed,
which correlated with the nonlinear behavior of the structural phase composition and lattice parameters with the change of
the composition.
The text was submitted by the authors in English. 相似文献
26.
A forecasting model is developed for the number of daily applications for loans at a financial services telephone call centre. The purpose of the forecasts and the associated prediction intervals is to provide effective staffing policies within the call centre. The model building process is constrained by the availability of only 2 years and 7 months of data. The distinctive feature of the data is that demand is driven in the main by advertising. The analysis given focuses on applications stimulated by press advertising. Unlike previous analyses of broadly similar data, where ARIMA models were used, a model with a dynamic level, multiplicative calendar effects and a multiplicative advertising response is developed and shown to be effective. 相似文献
27.
M I Salema A P B Póvoa A Q Novais 《The Journal of the Operational Research Society》2006,57(6):615-629
In this paper, an MILP formulation is proposed for the design of a reverse logistics network based on a warehouse location–allocation model, which optimizes, simultaneously, the forward and reverse networks. A single product model with unlimited capacity is first defined. Subsequently, the model is extended to a multi-product capacitated recovery network model, where capacity limitations and a multi-product system can be considered. The proposed model is compared to published work in the field, where different model assumptions have been proposed. Two cases are described so as to gain a better insight into the model and allow a comparative analysis. 相似文献
28.
Data Envelopment Analysis (DEA) offers a piece-wise linear approximation of the production frontier. The approximation tends to be poor if the true frontier is not concave, eg in case of economies of scale or of specialisation. To improve the flexibility of the DEA frontier and to gain in empirical fit, we propose to extend DEA towards a more general piece-wise quadratic approximation, called Quadratic Data Envelopment Analysis (QDEA). We show that QDEA gives statistically consistent estimates for all production frontiers with bounded Hessian eigenvalues. Our Monte-Carlo simulations suggest that QDEA can substantially improve efficiency estimation in finite samples relative to standard DEA models. 相似文献
29.
30.